Remove Tox reference to Python 3.6
Remove Tox reference to Python 3.6

This aligns testing with the list of supported versions in the setup.py
Python version classifiers list.

Signed-off-by: Alex Nelson <d5aca716d6ed55a86fa350cb5231ca56b21085ca@nist.gov>

RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib,RDFLib/rdflib